We evaluated a new lightweight capnometer with a less than 1 mL deadspace neonatal airway adapter and endotracheal tube connector unit (NAC) for use in mechanically ventilated neonates. The evaluation consisted of: 1) a bench test comparison of air flow resistance between the standard endotracheal tube and connector with the new NAC (flow rates, 1.5 to 12.8 L/min); 2) a determination of the effect of NAC placement on Paco2; 3) pre- and post-NAC pulmonary mechanics; and 4) analysis of paired PetCO2 and PACO2 in 16 infants requiring mechanical ventilation. Paired t test of the slopes of the resistance curves was significant (P = 0.002) while analysis of variance of differential pressures was not (P = 0.29). All post-NAC placement Paco2 were smaller than pre-placement values; there were no differences in pulmonary mechanics, and Petco2 correlated closely with Paco2 (n = 132, r = 0.79) defined as Petco2 = 0.68.Paco2 + 5.52; means +/- 1 SD, Paco2 -Petco2 was 4.7 +/- 4.7 torr and Petco2/Paco2 was 0.86 +/- 0.14.